Transformer power supply for microwave generators

ABSTRACT

Two ferroresonant transformers are provided to apply power to a magnetron, and each is adapted to resonate to provide half the power to the magnetron. The primary windings are connected in parallel across power lines, and a switch is provided to selectively connect one primary to the lines. In respective embodiments, both secondaries are adapted to operate through full-wave and half-wave rectifier means coupled to the anode of the magnetron, and in both the current drawn by the magnetron when the primary switch is closed is double that when the switch is open.

D United States Patent Inventor Helmut Boehm Thousand Oaks, Calif.

Appl, No, 810,671

Filed Mar. 26, 1969 Patented June 8,1971

Assignee Technology Instrument Corporation Newbury Park, Calif.

TRANSFORMER POWER SUPPLY FOR MICROWAVE GENERATORS 5 Claims, 4 DrawingFigs.

U.S. Cl 321/16, 219/1055, 321/20, 321/27, 323/61, 328/262 Int. Cl H02m7/04, 1105b 9/06, G05f3/06 Field 01 Search 321/16, 20, 27, 25; 323/60,61; 315/172; 328/258, 262; 219/1055 [56] References Cited UNITED STATESPATENTS 3,217,233 11/1965 Drusch 321/27 3,247,394 4/1966 Lowrance.....321/27X 3,371,263 2/1968 Waltz et al 321/25 3,470,942 10/1969 Fukada eta1. 2l9/10.55X

Primary Examinerwilliam H. Beha, Jr. AuorneyPerry E. Turner ABSTRACT:Two ferroresonant transformers are provided to apply power to amagnetron, and each is adapted to resonate to provide half the power tothe magnetron. The primary windings are connected in parallel acrosspower lines, and a switch is provided to selectively connect one primaryto the lines. in respective embodiments, both secondaries are adapted tooperate through full-wave and half-wave rectifier means coupled to theanode of the magnetron, and in both the current drawn by the magnetronwhen the primary switch is closed is double that when the switch isopen.

TRANSFORMER POWER SUPPLY FOR'MICROWAVE GENERATORS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates totransformer power supplies for microwave heating apparatus.

2. Description of the Prior Art In microwave heating apparatus such asovens for cooking foodstuffs, it is desirable to very the level ofenergy developed by the magnetron and injected into the oven cavity.Such an approach is desirable to permit one to selectively subject agiven foodstuff to different cooking speeds and energy levelsappropriate for optimum results. However, there is not presentlyknownsimple and reliable means for achieving this desired end.

SUMMARY OF THE INVENTION My invention embraces transformer power supplymeans with two power sections using constant current transformers,wherein a simple, conventional switch in one primary circuit is used tocouple one or both sections to a load which is in circuit with thesecondaries of both.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of apower supply of my invention using full-wave rectifier means in circuitwith the secondaries of constant current transformers;

FIG. 2 is a schematic diagram of a power supply of my invention usinghalf-wave rectifier means in circuit with the secondaries of suchtransformers;

FIG. 3 illustrates current waveforms to aid in explaining the operationof the circuit of FIG. I; and

FIG. 4 illustrates current waveforms to aid in explaining the operationof the circuit of FIG. 2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Referring to FIG. I, a pair ofconstant current transformers l0, 12 have their primary windings l4, 16connected in parallel across power lines 18, the terminals 20 of whichare adapted for connection to an AC source. As shown, a switch 22 isconnected in series with the primary winding 16. The switch 22 may be asimple single-pole, singleor double-throw switch of conventionalconstruction. As will be seen, this switch is all that is needed toselectively control the level of power applied to a magnetron,illustrated at 24, and thereby control the level or intensity ofmicrowave energy radiated from its output probe.

The secondary windings 30, 32 of transformers l0, 12 are coupled throughfull-wave rectifier means to the anode 34 of the magnetron. In theparticular arrangement shown, the upper end of the secondary winding isshown connected to the junction of two capacitor legs 36, 38 of afull-wave rectifier or bridge, and the lower end of such secondary isshown with a connection between diode legs 40, 42. The diode legs 40, 42in this example also serve as diode legs of a full-wave bridge connectedto the secondary winding 32, the lower end of which is shown connectedto the junction of capacitor legs 44, 46, and the upper end of which isshown with a connection between the diode legs 40, 42. An additionaldiode 48 is' connected between the diodes 40, 42, i.e., between thelower end of the secondary 30 and the upper end of the secondary 32.This latter diode 48 serves to isolate DC components of voltage andcurrent of each transformer from those of the other.

As shown, the junction 50 of capacitor 38 and diode 42 are connected tothe anode 34, which is connected to a point of reference or groundpotential. The junction 52 of the capacitor 36 and diode is connected toone leg of the filament 54 of the magnetron, such filament having itslegs adapted for connection in conventional fashion to the secondary ofa filament transformer (not shown).

The circuit of FIG. I will first be described with the switch 22 open asshown, and then with switch 22 closed. With such switch open, only thetransformer 10 is energized. Accordingly, the bridge constituted ofcapacitors 36, 38 and diodes 40, 42 functions as a full-wave rectifier,the positive and negative terminals 50, 52 of which are connectedbetween the anode 34 and the cathode 56 of the magnetron. Referring toFIG. 3 along with FIG. I, such rectifier develops current pulses 60 havea repetition rate double that of the frequency of the line voltage,e.g., 120 pulses per second for a 60 cycle line voltage. The magnitudeof current pulses 60 is determined by the values of the capacitors 36,38 and the turns ratio of the windings of the transformer 10. Suchcapacitor values and turns ratio preferably are designed so that thepower applied to the magnetron 24 via transformer 10 is one-half themaximum power to be applied thereto.

In similar fashion, the turns ration of the windings of the transformer12 and the capacitances of capacitors 44, 46 are designed so that whenthe transformer 12 is energized, current pulses developed by the bridgethat includes capacitors 44, 46 are of the same magnitude as currentpulses 60. Accordingly, when the switch 22 is closed to energize thetransformer 12, current pulses 62 are developed which are twice themagnitude of the current pulses 60. Therefore, the power applied to themagnetron when the switch 22 is closed is double that applied to it whenthe switch is open.

Referring to FIG. 2, the secondary windings 30, 32 are connected torespective half-wave rectifiers. As shown, a capacitor 70 and diode 72are connected in series between the ends of the secondary 30, and acapacitor 74 and diode 76 are connected in series between the ends ofthe secondary 32. The junction 78 of the capacitor 70 anddiode 72 areconnected to the anode 34 of the magnetron, and the junction 80 of thecapacitor 74 and diode 76 are connected to one leg of the filament 54.Also, the lower and upper ends of the secondaries 30, 32 are connectedtogether between the diodes 72, 76. Still further, the diodes 72, 76 areconnected in the forward direction between the junctions 80, 78.

Referring to FIG. 4 along with FIG. 2, let it first be assumed that theswitch 22 is open as shown, in which case only the transformer I0 isenergized. Due to half-wave rectification, current pulses 84 aredeveloped which have the same repetition rate as the line frequency. Dueto the value of capacitor 70 and the turns ratio of the windings oftransformer 10, the magnitude of current pulses 84 is such that thepower supplied to the magnetron via transformer 10 is one-half themaximum power to be applied thereto.

When the switch 22 is closed to energize the transformer I2, currentpulses 86 are developed which are of the same magnitude and repetitionrate as current pulses 84. However, current pulses 86 fall intermediatecurrent pulses 84. Further, the windings are so poled as to insure thesame polarities of the junctions 78, 80 for both current pulses 84, '86.When switch 22 is closed, therefore, the power applied to the magnetronis double that applied to it when the switch is open.

It is preferable that iron core transformers be used for thetransformers 10, I2, and that they be provided with means which duringoperation cause them to be driven to saturation in the vicinity of theirsecondary windings. An example of such a transformer is a pair ofC-shaped cores having their ends butted together, and having a magneticshunt extending between opposed portionsof the resultant structure onwhich the primary and secondary windings are placed. Magnetic shunts areindicated at 88, 90 for transformers l0, 12.

It will be apparent that my invention embraces the use of more than'twopower sections. For example, there may be three transformers with inputsof separate full-wave rectifiers connected to respective secondaries,and with the respective rectifier outputs tied together and toappropriate elements of the magnetron. In such case, each transformer isadapted to mers, depending on whether a third, two-thirds, or full poweris to be applied. Also, of course, ganging of such switches may beemployed to apply either two-thirds or full power as desired.

lclaim:

1. A power supply for a microwave generator comprising;

a pair of constant current transformers each having a primary windingand a secondary winding; means connecting said primary windings AClines,

said means including a switch in circuit with the primary winding of onetransformer and operably to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers;

and means coupling both pulse developing means to the generator, therebyto cause current pulses from both pulse developing means to be appliedto the generator when said switch is closed and to cause current pulsesfrom the pulse developing means in circuit with said other transformerto be applied to the generator when said switch is open.

2. A power supply for a microwave generator comprising:

a pair of constant current transformers each having a primary windingand a secondary winding;

means connecting said primary windings to AC lines,

said means including a switch in circuit with the primary winding of onetransformer and operable to selectively connect it to and disconnect itfrom the lines,

whereby both said one transformer and the other transformer areenergized when said switch is closed and only said other transformer isenergized when said switch is open;

means in circuit with each secondary winding for developingunidirectional current pulses from said transformers,

said pulse developing means including half-wave rectifier means,

each current pulse from each transformer when said switch is closedoccurring intermediate current pulses from the other transformer;

and means coupling both pulse-developing means to the generator, therebyto cause current pulses from both pulse developing means to beapplied tothe generator when said switch is closed and to cause current pulsesfrom the pulse developing means in circuit with said other transformerto be applied to the generator when said switch is open.

3. The combination of claim 2, wherein said half-wave rectifier meansincludes respective rectifiers each having a capacitor and a diode inseries with a respective secondary winding,

said diodes being directly connected in the same direction;

and a respective connection from the junction of each series-connectedcapacitor and diode, said connections being adapted for connectionacross the generator. 4. A power supply for a microwave generatorcomprising: a pair of constant current transformers each having aprimary winding and a secondary winding; means connecting said primarywindings to AC lines,

said means including a switch in circuit with the primary winding of onetransformer and operable to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers, including full-wave rectifier means, the currentpulses applied to the generator from both transformers when said switchis closed being coincident; said full-wave rectifier means including apair of bridges each having a pair of capacitor legs, the remaining legsof each bridge being two diode legs that form common legs for thebridges for both pair of capacitor legs; respective connections fromeach side of said isolation diode to one end of each secondary winding,the other ends of said secondary windings being connected respectivelyto the functions of the capacitor legs; and respective connections fromthe ends of said diode legs for connection across the generator. 5. Apower supply for a microwave generator comprising: a pair of constantcurrent transformers each having a primary winding and a secondarywinding; means connecting said primary windings to AC lines,

said means including a switch in circuit with the primary winding of onetransformer and operable to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers, including a pair of half-wave rectifiers each havinginput terminals connected to the ends ofa respective secondary winding,respective input terminals of said rectifiers being connected together;and means coupling both pulse developing means to the generator, therebyto cause current pulses from both pulse developing means to be appliedto the generator when said switch is closed and to cause current pulsesfrom the pulse developing means in circuit with said other transformerto be applied to the generator when said switch is open, includingoutput connections from both rectifiers connected across the generator.

1. A power supply for a microwave generator comprising; a pair ofconstant current transformers each having a primary winding and asecondary winding; means connecting said primary windings AC lines, saidmeans including a switch in circuit with the primary winding of onetransformer and operably to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers; and means coupling both pulse developing means to thegenerator, thereby to cause current pulses from both pulse developingmeans to be applied to the generator when said switch is closed and tocause current pulses from the pulse developing means in circuit withsaid other transformer to be applied to the generator when said switchis open.
 2. A power supply for a microwave generator comprising: a pairof constant current transformers each having a primary winding and asecondary winding; means connecting said primary windings to AC lines,said means including a switch in circuit with the primary winding of onetransformer and operable to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers, said pulse developing means including half-waverectifier means, each current pulse from each transformer when saidswitch is closed occurring intermediate current pulses from the othertransformer; and means coupling both pulse developing means to thegenerator, thereby to cause current pulses from both pulse developingmeans to be applied to the generator when said switch is closed and tocause current pulses from the pulse developing means in circuit withsaid other transformer to be applied to the generator when said switchis open.
 3. The combination of claim 2, wherein said half-wave rectifiermeans includes respective rectifiers each having a capacitor and a diodein series with a respective secondary winding, said diodes beingdirectly connected in the same direction; and a respective connectionfrom the junction of each series-connected capacitor and diode, saidconnections being adapted for connection across the generator.
 4. Apower supply for a microwave generator comprising: a pair of constantcurrent transformers each having a primary winding and a secondarywinding; means connecting said primary windings to AC lines, said meansincluding a switch in circuit with the primary winding of onetransformer and operable to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers, including full-wave rectifier means, the currentpulses applied to the generator from both transformers when said switchis closed being coincident; said full-wave rectifier means including apair of bridges each having a pair of capacitor legs, the remaining legsof each bridge being two diode legs that form common legs for thebridges for both pair of capacitor legs; respective connections fromeach side of said isolation diode to one end of each secondary winding,the other ends of said secondary windings being connected respectivelyto the functions of the capacitor legs; and respective connections fromthe ends of said diode legs for connection across the generator.
 5. Apower supply for a microwave generator comprising: a pair of constantcurrent transformers each having a primary winding and a secondarywinding; means connecting said primary windings to AC lines, said meansincluding a switch in circuit with the primary winding of onetransformer and operable to selectively connect it to and disconnect itfrom the lines, whereby both said one transformer and the othertransformer are energized when said switch is closed and only said othertransformer is energized when said switch is open; means in circuit witheach secondary winding for developing unidirectional current pulses fromsaid transformers, including a pair of half-wave rectifiers each havinginput terminals connected to the ends of a respective secondary winding,respective input terminals of said rectifiers being connected together;and means coupling both pulse developing means to the generator, therebyto cause current pulses from both pulse developing means to be appliedto the generator when said switch is closed and to cause current pulsesfrom the pulse developing means in circuit with said other transformerto be applied to the generator when said switch is open, includingoutput connections from both rectifiers connected across the generator.